1. Field of the Invention
The present invention relates to a signal processing apparatus.
2. Description of the Related Art
At present, apparatuses which reproduce voices and music, such as televisions or radio broadcast reception/reproduction apparatuses, music players, and portable telephones, are sometimes used in streetcars, in the outdoors, in automobiles, and in similar places where ambient noise exists. In this case, sound data which is reproduced by an apparatus (hereinafter, referred to as “sound data”) is masked by the ambient noise, depending upon the frequency or power relation between the sound data and the ambient noise, with the result that the clarity of the sound is lowered in some cases. In many sound reproduction apparatuses, sound data volume can be adjusted by a user. However, the sound volume adjustments cannot be made for the individual frequency components of the sound data. Therefore, the clarity of the sound is not always enhanced by increasing the sound volume. Besides, in a case where the sound data volume has been increased, the power of the whole band of the sound data is amplified. Therefore, the sound is sometimes distorted to a rather worsened sound quality. Further, when the sound volume is increased excessively, there is the possibility that the user's hearing will be damaged.
In this regard, there has been proposed, for telephone conversations in environments where there is ambient noise, a received voice processing apparatus wherein a frequency masking quantity and a time masking quantity ascribable to the ambient noise inputted from a microphone are calculated, and filtering for a received voice signal is performed by setting the filter coefficient of a digital filter on the basis of gains which have been determined for the respective frequency components of the received voice signal in accordance with the masking quantities, whereby even the sound masked by the ambient noise is amplified to an audible level (refer to, for example, JP-A-2004-61617).
According to the technique disclosed in JP-A-2004-61617, the whole band of the sound data is not amplified. Only the frequency component masked by the ambient noise can be amplified. In this case, a sound volume increment can be less than the increment of the sound volume when the whole band is amplified. The technique disclosed in JP-A-2004-61617, however, amplifies all the frequency components masked by the ambient noise. Therefore, a frequency component which is not sensed even when the ambient noise does not exist (a frequency component which is masked by another frequency component of the sound data) is also amplified, thereby unnecessarily increasing the sound volume. Moreover, an abnormal sound might be produced because the frequency component that is not sensed (because it is masked by the other frequency components) is amplified such that it is not masked by the ambient noise.